Template concave

The synthetic strategy used for the construction of concave pyridine bislactams 3 (Scheme 1) can also be applied to other concave bases. When instead of a pyridine-2,6-dialdehyde 4, l,10-phenanthroline-2,9-dicarbaldehyde (9) was used in a metal ion template directed synthesis of macrocyclic diimines, after reduction, also macrocyclic 1,10-phenanthroline diamines 10 could be obtained in good yields. Here too, the crude diamines 10 were used in the next reaction step. Bridging of 10 with diacyl dichlorides 8 gave concave 1,10-phenanthroline bislactams 11. Scheme 2 summarizes the synthesis and lists the synthesized bimacrocycles 11 [18]. 

Figure 7. Mechanism of catenane formation (amide type) the guest is orthogonally embedded in an intermediate macrocycle, the concave template. Depending on the substitution pattern of the reactants (pathways A and B) isomeric catenanes are obtained. For the sake of clarity the diacid dichloride is drawn here to be the nesting guest even though there is clear indication that the effective interactions take place between the corresponding monoamide and the macrocycle.

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). fn non-competitive solvents, the tetralactam macrocyde strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as wheeled nudeophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocyde not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel. 

Many of the proteins formed by expression of DNA genes also perform as templates. Enzymes, for example, are in fact concave templates (see later) and provide a complementary, enantioselective substrate environment or template cavity within which a chemical reaction takes place. Antibodies likewise bind to antigens selectively. Fidelity, selectivity and specificity of templating in biological systems are so far unsurpassed by man-made ones. Nevertheless very recently Mosbach et al. [4] were able to show that an immunoassay based on antibody recognition could equally well be conducted with an imprinted polymer. 

Scheme 3. Template synthesis by Vogtie and co-workers of amide-based catenanes with a concave template The catenation of the macromonocycle 15 with 13a and 14 leads to formation of catenane 17. The guest diacid dichloride 13a and the host 15 are locked in an orthogonal arrangement 16. The functional groups are fixed and thus preorganized for reaction with the diamine 14.

Scheme 5. Reaction between the educts 22 and 23 in CDCI3 in the presence of the concave template 21. (a) Schematic representation, (b) Specific template synthesis.

Further extension of the sol-gel and colloidal crystal templates include control of the outer shape of the colloidal crystal by assembling the PS particles in an aqueous droplet at the air/oil interface [23]. The assemblies then have regular shape (on the length scale of a few millimeters) spheres, ellipsoids, and concave disks. This was controlled by the addition of surfactant and an applied electric field. The cubic close-packed, ordered macroporous titania or silica obtained had similar outer shape as the template. 

Additional examples of tetraoxacuppedophanes based on the phenanthroline template are 159 and 160 with polymethylene chains as the bridging units, and the chiral concave base 161 [8]. 

The first two voliunes in this Templates in Chemistry series have focused on templates that control solution-phase reactions. Among the templates discussed in these two voliunes were convex and concave templates that mediate the formation of (macro)cyclic molecules and mechanically bound molecules with their intriguing intertwined topology. Also, three-dimensional templates that are used to imprint polymers and that organize compounds in the solid state for predestined reactions have been included in the earlier volumes. 

The field of dynamic combinatorial chemistry takes advantage of the template effect, and numerous macrocycles have been stabilized by a proper tanplate. Also for the construction of macrocycles for concave reagents, the template effect is very valuable. 

Already in the beginning of the history of concave reagents, the template effect of metal cations was utilized, but also organic molecules can be applied. Sanders for instance used the interaction of pyridines with zinc porphyrins in the construction of macrocyclic oligomeric porphyrins, and recently, the interaction between polyols and boronic acids has been used in order to synthesize new mdo-fimctionalized macrocycles (Figure 7.14) ... 

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